Harvesting power from renewable energy sources such as wind energy and solar energy is the key to global energy sustainability. Extracting power from renewable energy sources in an efficient and profitable manner has recently been a very active research area, due to the environmental concerns and diminishing fossil fuel reserves. Grid-connected DC/AC inverters are one of the key components in renewable energy power conditioning systems. The grid-connected DC/AC inverter is responsible for delivering the power extracted from the renewable energy source to the grid. In particular, the control system of the DC/AC converter is responsible for injecting a high quality current into the utility grid. Regulatory standards for interconnecting renewable energy sources with a utility grid (e.g. IEEE 1547) impose very strict requirements on the quality of the output current. In particular, there is a very strict requirement for the amount of DC current which can be injected to the utility grid through the DC/AC inverter (less than 0.5% as imposed by IEEE 1547).
Proportional-Resonant (PR)-controllers are commonly used in controlling the output current of the DC/AC inverter with an inductive filter at the output (prior art). If a third-order LCL-filter is used at the output, a PR-controller along with a linear state-feedback is used to control the output current and to damp the resonance created by the LCL-filter (prior art). PR-controllers are unable to eliminate DC current from being injected into the grid since they cannot provide very high feedback gain for DC signals. Therefore, limiting the injected DC current is very challenging when using PR controllers.
From the above, there is therefore a need for methods and systems which can reduce if not eliminate the DC current from the output of grid-connected inverters.